In recent years, computer systems have exhibited major advances in speed and in miniaturization at significant reduction in cost. Concurrent with these advances have been major efforts to develop information storage and retrieval systems which are also low cost and still compatible with the high speeds with which these new systems operate. Much of this effort has been directed toward the development of optical memory devices because of their rapid write time (&gt;1 Mbit per second), rapid access time (&lt;0.1 sec.), high density ( 10.sup.8 bits per sq. in.), and low cost (&lt;10.sup.-4 cents per bit). (See Kenney, et al., IEEE Spectrum, pages 33-38, February 1979.) To date, numerous types of materials have been developed for optical discs; however, most of these have the disadvantage of storing information permanently; i.e., they cannot be erased or edited. (See Bartolini, et al., Optical Engineering, 15(2), 99 (1976).)
Materials which are exceptions to this general rule appear to fall into the following classifications: certain thermoplastics, photochromics, chalcogenides, magnetooptical materials, photoferroelectric materials, photoconductive/electrooptical materials and electrooptic materials. However, each of these has significant disadvantages. For example, thermoplastics such as polyvinylcarbazole/polystyrene require a pre-exposure corona charge. In addition, they use surface relief storage which has a relatively low contrast ratio, a long erase time, difficulties in local erase, and a short lifetime (typically .about.100 cycles). Photochromics such as spiropyrane typically require blue or ultra-violet light for write and/or erase and hence are not compatible with present semiconductor lasers. Furthermore, most photochromics are subject to fatigue, which severely limits cycle lifetime, and the stored data tends to fade in just a few minutes. Chalcogenides such as TeAsGe typically exhibit a low cycle lifetime (.about.10 cycles) and have a relatively low contrast ratio for erasable media. Magnetooptical materials such as the rare earth iron garnets usually require an external magnetic field. Materials with the largest magnetooptical effects (garnets) require micro-patterning to get high density storage and have high optical insertion loss. Photoferroelectric materials such as Bi.sub.4 Ti.sub.3 O.sub.12 require single crystals which are difficult to prepare in large areas. Large area photoferroelectrics can be fabricated as ceramic materials but the ceramics are subject to fatigue. Photoconductive/electrooptic materials such as Bi.sub.12 SiO.sub.20 have limited data storage times, on the order of several hours, and also require large single crystals. Electrooptic materials such as LiNbO.sub.3 also require single crystals and stored information is erased during readout unless the image is thermally fixed whence another thermal treatment is required for erasure.
The use of liquid crystal materials, particularly smectic liquid crystals is well known in the prior art for certain display devices, and some stationary memory systems have been developed which use this media for information storage. (See U.S. Pat. No. 3,796,999 entitled LOCALLY ERASABLE THERMO-OPTIC SMECTIC LIQUID CRYSTAL STORAGE DISPLAYS and Dewey, et al., STD 77 Digest 108 (1977).) However, despite continued research over the last ten years to achieve ever higher information density in liquid crystal light valves, the perceived practical limitations of these devices have all but eliminated their widespread use in electronic data processing memory applications where, to be competitive, information densities of the order of 10.sup.7 bits/cm.sup.2 are required. One of the primary reasons for this is that it has generally been believed that the spatial resolution of electro-optical effects in these devices is of the same order of magnitude as the cell thickness (see J. D. Margerum and L. J. Miller, J. Colloid and Interface Sci. 58, 3(1977)). Furthermore, independent analytical and experimental work has tended to confirm this perception in the specific case of thermally addressed smectic light valves, where the information is encoded in the form of radiation scattering defects in an otherwise non-scattering background. (See A. G. Dewey, "Projection Storage Displays Using Laser-Addressed Smectic Liquid Crystals," in The Physics and Chemistry of Liquid Crystal Devices, G. Sprokel, ed., Plenum Press, 1980, Pgs. 219-238.) For such defects to be useful in carrying information, the prior art would indicate that these defects would have to be essentially independent, i.e., a disturbance of one defect does not destroy information carried by adjacent defects. Hence, it would be expected that the highest useful defect density would be obtained with an array of isolated defects, each defect corresponding to a single domain. Further, for high resolution and a high quality non-scattering texture, the smectic-A liquid crystal should be homeotropically aligned in the bulk. Under these conditions, it would be expected that the single domains thus created would correspond to the parabolic focal conics discussed by Rosenblatt, et al., J. Physique 38(a), 1105 (1979). These parabolic focal conics form domains extending through the cell thickness and exhibit polygonal arrays on the cell surface. The size of these domains would define the defect density and for the special case considered in Rosenblatt (id. at 1110) the focal length, f, the cell thickness, t, and the domain width, d (the width of the parabola at the surface), can be shown to be related by the following expression: EQU f=d.sup.2 /8t
Experimental results on smectic-A samples of cyanobenzylidene octyloxyaniline (CBOOA) exhibited focal lengths ranging from 1.3 .mu.m to 2.9 .mu.m, which when used with above analytical expression to estimate domain sizes, corresponds to a range of approximately 11 .mu.m&lt;d&lt;17 .mu.m for a 12 .mu.m thick layer, and 4 .mu.m&lt;d&lt;7 .mu.m for a 2 .mu.m thick layer, or more generally d.about.t. In addition, using smectic materials other than CBOOA, for example, leads to substantially the same results.
In terms of information density, a domain width of 11 .mu.m corresponds to approximately 10.sup.6 bits/cm.sup.2 and a domain width of 4 .mu.m corresponds to approximately 10.sup.7 bits/cm.sup.2. Hence, to achieve information densities suitable for optical computer memory applications requires very thin liquid crystal layers, of the order of 2 .mu.m or less. Although such cells have been produced, (see A. D. Jacobson, et al., SID Int. Symp. Dig. 6, 26 (1975)), the practical problems associated with maintaining uniformity of spacing at such thicknesses have severely restricted their use. As an example of these problems, 0.5 inch thick polished optical flats have been required as substrates just to maintain uniformity of spacing over fields as small as 1 in..times.1 in. for 2 .mu.m thick cells. Not only are such cells expensive to produce, but such substrate thicknesses can seriously interfere with the resolution of the optical systems which are required to achieve the small spot sizes needed for high density storage. Furthermore, in large area cells, uniform liquid crystal layer thickness is quite difficult to achieve with just perimeter type spacers even with liquid crystal layers as thick as 25 .mu.m. Hence, special techniques have had to be developed to insure uniformity, including, for example, the addition of small glass beads of the appropriate size to the liquid crystal medium, and insertion of intermediate spacers.
For all of these reasons, and despite large research and development programs continued over many years by many companies, practical inexpensive liquid crystal light valves have not been developed with information densities approaching 10.sup.7 bits/cm.sup.2, densities such as would be practical in electronic data processing memories or in very high resolution display devices.